Generally, carbon nanotube (carbon nanotube; CNT) is formed from hexagonal arrays of carbon atoms with a sp2 bond and a structure thereof is a honeycomb-shaped network of graphite sheet rolled into a cylindrical shape or tube. Carbon nanotube has a diameter of from a few □ to a few tens of nanometers (nm) and its length ranges from several tens to several thousands times greater than that of its diameter.
Such carbon nanotube has a structure of a graphite sheet rolled into cylindrical shape and the properties of carbon nanotube vary greatly depending on its structure and size. It has ultimate high strength of 5Tpa, which is greater than those of metals [J. Mater. Res. 1998, 13(9), 2418]. Further, it can exhibit the properties of insulators, semiconductors, or metals according to its structure and diameter, factors which are known to change electrical properties.
Since graphite sheets helically coil around nanotube to form carbon nanotube, electron movements are also changed to form an armchair or a zig-zag patterned structure when the direction of the helix is changed.
There are two main types in carbon nanotubes: single-walled nanotube (SWNT) and multi-walled nanotube (MWNT). In tire industry, it is expected that carbon nanotube would replace carbon black since the composite surface is much softer than that of carbon black or carbon fiber and it rarely loses fragments. Especially, more than 80% of silica cannot be filled due to static electricity in manufacturing tires but more than 95% of silica can be filled when carbon nanotube is used [Polymer Science and Technology 2005, 16(2), 162]. It is also disclosed that a composite of carbon nanotube and polystyrene enhances more than 40% of elasticity and tensile strength with only 1% amount [Polymer Science and Technology 2005, 16(2), 176].
There are methods for improving dispersion and interfacial property with matrix via physical treatment utilizing non-covalent bonds and inducing covalent bond by chemical modification of carbon materials.
When lanthanum series metals, i.e., metals from atomic number 57 of lanthanum to 71 of lutetium, are used as a catalyst in the preparation of polybutadiene via 1,3-butadiene polymerization, it provides polybutadiene with relatively higher 1,4-cis content as compared with those of transition metals such as nickel, titanium and cobalt. Of lanthanum metals, cerium, lanthanum, neodymium, and gadolinium show better catalytic activities, and neodymium the best.
In WO 97/36850, WO 98/39283, UK Pat. No. 2,140,435, EP Pat. Nos. 512,346 and 599,096, U.S. Pat. Nos. 5,428,199, 5,449,387, and 5,360,898, and Polymer (1985, vol 26, p 147), there are disclosed methods of preparing polybutadiene by using lanthanide catalyst, prepared with lanthanide chloride, lanthanide nitrate, lanthanide oxide or neodymium carboxylate. Of the lanthanide compounds, neodymium carboxylate is most effective.
Such active rare earth catalysts are generally prepared from a neodymium compound, an organoaluminum and a halogen compound.
However, the catalytic activity of neodymium obtained by the above methods is no more than 7%, and the low activity usually results in forming a gel. Especially, it is difficult to remove the salts such as nitrates, chloride, and sulfates contained in the neodymium compound. Further, solvents including water, alcohol, ether, dimethyl formamide and the like used in the preparation of neodymium compounds coordinate with neodymium, which thereby accelerates coagulation and reduces the efficiency of the catalyst [Polyhedron 1989, 8(17), p 2183; J. Mater. Chem. 1998, 8, p 2737].
EP Pat. No. 11184 and U.S. Pat. Nos. 4,260,707 and 5,017,539 dislcose methods for preparing high 1,4-cis polybutadiene by using neodymium carboxylate, for example, the preparation of high 1,4-cis polybutadiene by using a catalyst comprising neodymium carboxylate, alkyl aluminum compound, and Lewis acid in the presence of a non-polar solvent.